The present invention relates to an arrangement for contact arc ignition in manual TIG welding.
In TIG welding today, there are principally three different methods for establishing the arc.
Contactless ignition by means of the protective gas being ionized by high-frequency high voltage. This method is called HP ignition and is regularly used in high-quality welding, where it is important that no inclusions of the electrode material are present in the weld deposit. The method provides good starts, but it also has disadvantages such as a high cost and often very disruptive radiation. This means that the method cannot be used in electrically sensitive environments.
Contact ignition according to the LIFTARC(copyright) ignition method in which the electrode is brought into contact with the workpiece. Thereafter the welding gun trigger is pressed and the current source delivers a low current until the gun is lifted and an arc voltage can thereby be detected. The current is then raised automatically to the value set for the welding. The aim of the low starting current is to reduce inclusions of the electrode material in the weld deposit. The method gives good results where welding is done under stable conditions, i.e. with a well clamped workpiece and constant positioning of the welder. If these two requirements are not both satisfied, it is easy to inadvertently cause dipping of the welding electrode immediately after the arc has ignited. Since the current has then been raised and a molten pool is under formation, the electrode will adhere or stick firmly in the small melt, which rapidly solidifies when the arc goes out. This leads to undesired inclusions of electrode material in the weld deposit and also to operational stoppages, since the electrode has to be reground. The quality in TIG welding is to a large extent dependent on the electrode being well ground with the correct angle at the tip. Otherwise, the arc stability is negatively affected, as is the penetration in the seam. The LIFTARC(copyright) ignition method also requires a great deal of electronics in the welding machine since the arc voltage has to be measured. This is particularly so if the control part of the current source lies on the primary side in an inverter current source. In that case, a galvanic isolation of the analogue arc voltage must take place before it can be used in the control part. Alternatively, dual electronics systems with dual voltage supplies, etc., can be constructed. This makes such current sources more expensive.
Finally, there is the third and simplest ignition methodxe2x80x94scrape ignition. The method involves the welder very briefly stroking or dipping the electrode against the workpiece while the current source is set at full welding current. The method demands great skill, and in all circumstances involves a considerable amount of wear of the electrode, with frequent regrinding as a consequence. Obviously this is the method which gives the greatest inclusions of electrode material in the weld deposit. The advantage of the method is that it does not necessitate any extra equipment in the machine. Only a TIG torch with gas valve is needed in order to use a conventional MMA machine, primarily intended for welding with coated electrodes, as a TIG power source. A problem in this connection is the start surge which an MMA machine is provided with. At each start, the current is increased in relation to the set current value for 200-700 milliseconds. This is an important function in order to quickly form a molten pool so that the coated electrode does not stick firmly. At least at higher currents, the amplitude of the start surge is directly dependent on the set current. In the case of TIG welding with scrape starting, this can easily lead to overloading of the electrode. In addition, a molten pool is very quickly formed which, in the case of dipping of the electrode on account of shaking at the start of welding, often leads to the electrode sticking, which entails regrinding and lost productivity. Another problem with the method arises when the arc is to be interrupted. This is done by the welding torch being removed from the workpiece to a sufficient extent to ensure that the arc will go out. In welding at a high current, this is a problem, since the arc can in these cases be a good 50 mm before it is interrupted.
The object of the invention is to provide an arrangement for contact ignition of a TIG arc, which affords a solution to the above problems without having the inherent disadvantages of the prior art.
According to the invention, this object is achieved by using a switch on the welding gun to shift manually between a low ignition current and a set welding current. Thus the electrodes is held in contact with the workpiece with such a low current that the electrode is not damaged. This contact time is not in any way critical. Thereafter the torch is lifted and an arc is established with the low ignition current. The arc is stable with respect to great variations in the arc length. 10 mm does not cause any problems. As long as the current is low, the basic material is not melted appreciably if the material thickness is greater than 0.7 mm, and the torch can therefore be moved on the workpiece without leaving any troublesome traces. It is only when the electrode is situated in the correct starting position, and when the welder sees fit, that he presses the welding gun trigger. The current then rises to the set welding current. The method eliminates the critical moment when the arc has just been ignited and the molten pool is beginning to take form, while the electrode is still situated in immediate proximity to the workpiece.
The invention also affords considerable advantages as the welding proceeds. By releasing the gun trigger and thereby going down to the low ignition current, the welder can compensate for excessively high heat, for example due to varying gap conditions, without having to interrupt the arc and reignite it. This can also be made use of between different seams, as well as at the end of a seam, in order to successively reduce the supply of heat and thereby reduce the size of the molten pool in order to reduce the risk of an end crater upon completion of welding. With a conventional MMA current source, an extra remote device is needed for this control. In TIG welding with filler metal, the welder has both hands occupied and does not therefore have the possibility of manually handling a additional remote device. In this connection, the invention clearly affords better handling.
Electrode wear is dramatically reduced, on the one hand as a result of more reliable starting, with greatly reduced risk of electrode dipping, and on the other hand as a result of the possibility of avoiding the need to interrupt the arc during welding, both in one and the same weld bead and also between different beads on the same workpiece.
A further advantage is that it is possible to improve the penetration by means of welding at a higher welding current than is recommended for continuous welding, since it is easy to reduce the heat supply by releasing the trigger.
At the end of welding, the arc is of course interrupted from the low current level, which eliminates the very long arcs associated with scrape starts.
The arrangement does not need any extensive electronicsxe2x80x94no complicated galvanic isolation of analogue signals, when the control electronics lie on the primary side in inverter machines. This provides cost advantages compared with the LIFTARC(copyright) method.
Compared with HF starts, the invention has, in addition to cost advantages, the further advantage of not emitting disruptive radiation, which means that it can be used in environments where HF-started machines are not approved.